Motion sensors have been widely used in a variety of applications including automotive control systems. Examples of such automotive applications include anti-lock brake systems, active suspension systems, active occupant restraint systems such as air bags and the like, and vehicle impact sensing systems, to name a few. In any of the foregoing systems, angular rate sensors are generally used to sense rotation of an automobile while accelerometers are generally used to sense acceleration/deceleration of an automobile.
In the past, electromechanical and electronic motion sensors or accelerometers have been used in automotive applications to detect automobile acceleration/deceleration. More recently, sensors that employ an electrically-conductive, micromachined plated metal or silicon sensing element have been developed which can be integrated with known semiconductor wafer processing techniques such as, for example, bipolar, CMOS, and BiCMOS processes. An example of a plated metal surface micromachined sensor is disclosed in U.S. Pat. No. 5,450,751 to Putty et al. which is assigned to the assignee of the present invention, and the contents of which are incorporated herein by reference. The Putty et al. device is formed in accordance with a metal plating technique in co-operation with a mold that defines a resonating ring and spring system affixed to the surface of a wafer. One variation of the Putty et al. sensor is disclosed in U.S. Pat. No. 5,547,093 to Sparks, which is also assigned to the assignee of the present invention and which is also incorporated herein by reference. The Sparks device is an electrically-conductive, micromachined silicon sensing element that is formed by etching a "sensing" chip from a single-crystal silicon wafer or polysilicon film on a silicon or glass carrier.
The Putty et al. and Sparks sensors each include a number of capacitive sites disposed about the perimeter of the ring structure, wherein the various capacitive sites serve as electrode interfaces to the sensor. Conductive runners on the sensing chip enable the electrodes to be electrically interconnected with appropriate signal conditioning circuitry and to provide a biasing voltage to the ring. In operation, some of the electrodes serve as "drive" electrodes that drive the ring to resonate when these electrodes are appropriately energized. Other electrodes serve as "balance" electrodes that, when energize, serve to balance the resonant peaks of the flexural movement of the ring by changing the electromechanical stiffness of the ring and springs. Still other electrodes serve as "sensing" electrodes that capacitively sense the proximity of the ring relative to these sensing electrodes. With the foregoing construction, the sensor is able to detect movement of the ring vibrational pattern toward and away from the sensing electrodes, which occurs in response to the angular velocity of the ring about its axis of rotation due to effects of the Coriolis force. Thus, when appropriately installed, the sensor is operable to sense rotation rate about any chosen axis of an automobile.
Sensors of the type just described are capable of precise measurements and are therefore desirable for use in automotive applications. However, the operation of such sensors can be adversely affected by certain environmental operating conditions as well as certain external stimuli. For example, a sufficiently large gap must exist between the electrodes and the sensing element ring to prevent shorting, yet this gap must also be sufficiently small to maximize the capacitive output signal of the sensor. Temperature sensitivities exist due to the narrowness of the gap required between the ring and the sensor's drive, balance and sense electrodes, the effects of which are compounded by the large length ratios between the ring and the electrode structures. The natural frequency of the ring is also affected by temperature, which can impact the scale factor response of the ring at resonance. U.S. Pat. No. 5,872,313 to Zarabadi et al., which is assigned to the assignee of the present invention and the disclosure of which is incorporated herein by reference, discloses a variation of the Putty et al. and Sparks sensor wherein the electrode structures are configured to reduce the sensitivity of the sensor to temperature variations.
As an example of the adverse affects of certain external stimuli on sensor operation, it is paramount that the level of vibration on the sensing element of the type just described be controlled while rejecting unwanted sources of vibration such as, for example, road vibration. Heretofore, typical road vibration rejection strategies involved the design and implementation of shock-absorbing sensor mounting structures. However, such mounting structures are generally bulky and expensive to implement. What is therefore needed is a technique for electronically compensating a ring-type angular rate sensor, as this type of sensor is described hereinabove, to thereby reject unwanted road vibrational effects.